Ultrawide bandwidth (UWB) signals allow large amounts of data to be sent very rapidly at very low power. Data in a UWB signal is generally sent in one or more very narrow duration (i.e., very high frequency) wavelets (also called chips). It is desirable to arrange these wavelets in such a way as to increase the speed of operations while reducing the signal power as much as possible.
UWB wavelets can be generated in a variety of different manners. In some embodiments they can be Gaussian monopulses. In others, they can be repeated cycles of a sinusoid. In still others they can have different desirable forms.
FIG. 1 is a graph of a UWB wavelet stream using monopulse wavelets according to a disclosed embodiment of the present invention. Here the pulse is a Gaussian monopulse with a wavelet period Tw (also called a chipping period) of several nanoseconds, and a bandwidth of several gigahertz.
One embodiment of a UWB system uses signals that are based on trains of short duration pulses for the wavelets. These wavelets are formed using a single basic pulse shape. The interval between individual pulses can be uniform or variable, and there are a number of different methods that can be used for modulating the pulse train with data for communications.
In this embodiment these individual pulses are very short in duration, typically much shorter than the interval corresponding to a single bit, which can offer advantages in resolving multipath components. We can represent a general UWB pulse train signal for this embodiment as a sum of pulses shifted in time, as shown in Equation 1:
                              s          ⁡                      (            t            )                          =                              ∑                          k              =                              -                ∞                                      ∞                    ⁢                                    a              k                        ⁢                          p              ⁡                              (                                  t                  -                                      t                    k                                                  )                                                                        (        1        )            
Here s(t) is the UWB signal, p(t) is the basic pulse (i.e., wavelet) shape, and ak and tk are the amplitude and time offset for each individual pulse. Because of the short duration of the pulses, the spectrum of the UWB signal can be several gigahertz or more in bandwidth.
FIG. 2 is a graph of a UWB wavelet stream using repeated cycles of a sinusoid as wavelets according to a disclosed embodiment of the present invention. As shown in FIG. 2, each wavelet is formed from three consecutive cycles of a sinusoidal signal. As a result, in this embodiment the wavelet period Tw (also called the chipping period) is three times the period of the sinusoid, i.e., the wavelet has a wavelet frequency Fw (also called a chipping frequency) that is ⅓ the frequency of the sinusoid. However, although three repeated cycles are used to form a wavelet, alternate embodiments could vary the number of cycles used.
As shown in FIGS. 1 and 2, for binary encoding of data, the wavelets can be modulated into either an inverted wavelet 110 or 210 or a non-inverted wavelet 120 or 220.
One common characteristic in these embodiments is that the wavelet train is transmitted without translation to a higher carrier frequency, and so UWB transmissions using these sorts of pulses are sometimes also termed “carrier-less” radio transmissions. In other words, in this embodiment a UWB system drives its antenna directly with a baseband signal.